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Running a 6 Speed Box on a 3.92 Diff


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Now getting used to a K series having had a BEC for the last three years I have a few Q's I need to answer, so here's the first (having spent hours browsing Tech Talk already).


My new car is a 2004 Tracksport (140BHP [albeit upgraded by Minister to 160ish] k Series) which was built with a 5 speed box running on a 3.92 diff (with LSD) I think. The previous owner fitted a 6 speed box though I believe didn't change the diff ratio.


From the very limited miles I've put on the car so far I'm convinced either the speedo is reading way out or the car is a damn site faster than I was expecting.


I've read plenty of TT threads to see that people do run with the 3.92 diff on a 6 speed box albeit that the 3.62 is CC choice for new builds.


So question is, with a 5 to 6 speed box swap (on a 3.92 diff) is the speedo reading affected?


Chris *thumbup*

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The gearbox output shaft will be turning at the same speed for any given road speed.

I suspect the speedo drive ratio is different.

Some speedos have a set of calibration switches on the back. A sat-nav wirh speed is handy for getting a GPS soeed to set it up.


Good luck, an acurate speedo allows you to go faster!




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Hi Chris, I cannot help on the speedo reading but I run a 6 speed with a 3.92 lsd.

It took some getting used to going from a 5 speed VVC but now that I have I wouldn't go back. Motorway driving requires ear protection but then I used this with the VVC anyway. Be interesting to see how you feel this one compares to your blackbird

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What does your speedo say at 7400rpm in 6th gear? If it's between 120 and 125mph then it's about spot on *thumbup*


Have a search for Gearcalc, you can enter your gearbox and diff ratios and it will come out with the road speed.

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Cheers All,


Dignity, Wheels are 13" with CR500s (205/55/13 on rears).


Peter, Sat Nav's on standby for next outing.


Paul, will let you know, feels very rapid already though gears very short (feels odd being limited to 7000rpm-ish when the last few years have been 7-11,000 😬)


I wonder if the speedo gear has 22 teeth (as would have been fitted to a standard 6sp/3.62 setup?) rather than 24 as the 5sp might have had?





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I had this set up when I bought my car with 6 speed box (great) and 15" prisoner wheels (disaster), have learnt alot since 1997!!!!

Changed to 13" and what a joy, could use decent tyres and drop in unsprung weight made handling great.

Changed to 3.62 dif. lsd, and then the speedo was only about 12% fast. With 3.9 and 13" 205x60 was at least 18% fast.

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Kriss, I'm not sure what vintage your car is but on my 99 S/L my speedo over reads by approx 20%. There are no settings that I could find to change on the speedo, so I fitted a bike computer as an additional readout, these are programmable to the size of your actual wheel/tyre combination so are 100% accurate.



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